The present invention is directed to what may be termed flowable cementitious or grout slurries which are useful as fills, which, for example, require lower compressive strength in the range of 5 p.s.i. to 2,000 p.s.i. Typically, the most common fill material mixture comprises an acqueous slurry of cement, coal fly ash, water, and perhaps some other inert fillers. This slurry material, when cured and hardened, has sufficient structural strength to be useful for many purposes. More recently, a slurry utilizing cement, coal fly ash and water, with the coal fly ash being the main ingredient in the ratio of 75% or more, is becoming a fill mixture of choice because the United States Environmental Protection Agency under the Resource Conservation Recovery Act has mandated the use of coal fly ash in concrete for federally funded projects utilizing a designated quantity of concrete, if the coal fly ash--concrete alternative is economically and structurally viable. This mandate was legislated in the United States because of the high cost of land filling the tremendous quantities of coal fly ash, which is the by-product of coal combustion, particularly for the production of electricity.
In the past, the vast majority of such flowable slurries have been produced at ready mix concrete plants and transported to the job site in ready mix concrete trucks. The number of available suppliers of such material was limited to ready mix concrete plants that had the required storage facilities for both cement and coal fly ash. The normal practice was for the cement and coal fly ash to be delivered to the appropriate plant silo via trucks. Then, to be used at a job site, the cement and fly ash were transferred from such storage bins or silos to hoppers, where each was weighed out in a batch type operation to obtain the desired proportions of material. Thereafter, water was added and the materials were completely mixed, either in a concrete drum mixer at the ready mix transfer plant, or in ready mix concrete trucks which delivered the material to the use site. The process was not cost effective for a number of reasons, including the requirement for the ready mix concrete plant to proportion the cement, fly ash and water, the factor that the cement and coal fly ash were transported twice, once to the ready mix concrete plant and thence to the job site, the fact that the ready mix concrete truck could haul only perhaps a maximum of ten cubic yards at one time, thereby requiring far too numerous trips to supply a single project, and finally the requirement that the ready mix concrete truck had to be completely washed after delivery of its fill so as not to contaminate any concrete transported.
Another method of producing the flowable slurry involved mobile concrete mixer trucks which mounted separate hoppers for coal ash and cement, and proportioning and the use of mixing equipment to mix the two products with water to produce a flowable fill at the job site. This use of this type of equipment has been limited, since only a relatively minimum amount of material can be stored in the truck hoppers at any one time, the cement and coal ash are not loaded pneumatically but must be loaded from silos, and the coal ash and cement cannot be loaded simultaneously with the production of slurry.
Finally, in recent years, prior art systems have been used at the job site which utilize separate adjacent fly ash and cement bins feeding a conveyor through metering valves with the dry products being conveyed together, and then mixed and treated with water to produce the flowable slurry. While each of these bins was separately loadable pneumatically during the production of slurry, and a separator was later used to remove the solids from the loading air before releasing it to atmosphere, various problems were encountered which the present invention has solved. Most of the problems encountered were related to the lack of consistency of the end product with respect to the relative proportions of fly ash and cement which were present in the end product and dictated the compressive strength of the cured and solidified product. For example, when the old system was delivering product at a rate of one ton per hour, the relative proportions of ingredients could be off 30% with the result that the slurry continuously supplied to the project varied between 170 p.s.i. and 1,000 p.s.i. in compressive strength.
The present invention is directed to the improvements which have been made in the foregoing system to enable the output of slurry to have proportions within 3% to 4% of the desired proportions in a consistent manner, and to furnish a homogenous slurry which will have the required compressive strength at the twenty-eight day measuring period.